Polyisocyanates based on tolylene diisocyanate (TDI) are used commercially for surface coatings and adhesives as crosslinkers in 2-component polyurethane systems. Their purpose is to effect chemical crosslinking of isocyanate-reactive components, e.g. polyols, and curing to give a chemicals-resistant and mechanically strong film. Mixtures of urethanized adducts and isocyanurates of TDI are often used for this purpose.
Isocyanurates of TDI are prepared by cyclotrimerization using various catalysts. Reaction products of this type have been known for a long time and are described, for example, in DE 951168 B, DE 1013869 A, U.S. Pat. No. 6,936,678 B2, DE 19523657 A1, U.S. Pat. No. 4,255,569 A, EP 2174967 B1 and CN 105001701.
There has long been a desire to prepare the known isocyanurates of TDI firstly with a low viscosity and secondly with a high functionality.
A low viscosity is desired, for example, in order to improve the application behaviour of surface coatings and adhesives. In addition, use of low-viscosity polyisocyanates as crosslinkers of surface coatings and adhesives enables the solvent content of the formulation to be reduced. This means that the emissions of volatile organic compounds from such formulations can be reduced without having an adverse effect on the usability.
Furthermore, it is desirable when using such polyisocyanates as crosslinkers in surface coatings and adhesives for the polyisocyanates to have a high content of isocyanate groups. This further increases the sustainability in the sense of a low content of organic solvents and rapid crosslinking, i.e. high process efficiency.
In addition, it is desirable for the isocyanurates of TDI to have a low content of free diisocyanate. Owing to the toxicological concerns about monomeric TDI, this is an important condition for universal usability in industrially applied surface coatings and adhesives.
The reaction of TDI to form the polyisocyanurate gives, as is known, for example, from DE 951168 B and DE 1013869 A, very highly viscous resins, which makes processing more difficult or makes it necessary to use larger amounts of organic solvents. In addition, polyisocyanates of TDI have a high tendency to crystallize and are only sparingly soluble in organic solvents.
Owing to the high viscosity, even at high temperatures, it is, according to the prior art, not possible to free such resins of monomeric TDI easily by distillation. U.S. Pat. No. 4,255,569 A, EP 2174967 B1 and CN 105001701 describe various ways of carrying out a work-up by distillation in order to reduce the content of monomeric TDI by chemical modification or the addition of distillation auxiliaries. Since such additives reduce the content of functional groups (calculated as proportion by weight of isocyanate groups (NCO groups) based on the total weight of the composition), this is undesirable in principle.